Field of the Invention
Embodiments of the present invention generally relate to a method of injecting a sealant into a hydrocarbon well.
Description of the Related Art
Wells used for the recovery of hydrocarbons from subsurface formations which are drilled off shore, i.e., over water, and completed with wellheads located at the sea bed are known as subsea completions or riserless completions. All deepwater wells drilled in U.S. waters today are completed with subsea completions. Over time, as the well stops producing hydrocarbons in sufficient quantities to justify continuing to maintain the well, the well may be abandoned. Additionally, wells may need to be reworked, or repaired. In both cases, sealant will be injected into the well at a desired location within the well to temporarily or permanently seal the producing region of the well from the wellhead.
Accessing these wells for such remedial or abandonment operations is difficult. Initially, access to the well must be provided, typically by removing the subsea completion hardware. Additionally, a conduit must be lowered from a floating vessel dynamically positioned in place over the well to supply the sealant from a surface vessel into the sealant placement location within the well. This is a complex, costly operation requiring not only a specialized vessel but sufficient well control equipment to establish connection from the vessel to the subsea wellhead. The vessel, once connected, must remain positioned over the well while plugging sealants are mixed and pumped into the well; an operation requiring days or weeks to complete.
An alternative abandonment method would allow access to the well through existing production lines traveling from the subsea wellhead to the fixed production facility at the water's surface. These production lines carried hydrocarbons from the wellhead to the production facility throughout the well's producing lifetime. These lines are usually large diameter, long (50,000 feet is not uncommon) and can lie in a convoluted path. After years of production, the condition of flow lines is not suitable for sealant placement due to potential for hydrocarbon residue or other deposits in the lines that can disrupt flow or contaminate the sealant.
Even if the production lines were suitable for sealant transport, Portland cement slurry, the current sealant of choice for the petroleum industry, presents several impediments to placement via flow lines. First, cement slurry exhibits significant incompatibility with hydrocarbon fluids resulting in chemical reactions therebetween. This chemical reaction can produce highly-gelled mixtures that have high viscosity and a tendency to cling to the walls of the production line, which as a result will require excessive pressure to push the Portland cement sealant through the flow path, or will block flow through restrictions in the flow path such as valves or wellheads. The incompatibility can extend the hydration reaction of the cement, i.e., hardening, producing a cement that hardens slowly and develops reduced strength or does not harden at all. In addition to potential compatibility issues with contaminants that might be present in the flow line, Portland cement slurry stability poses an obstacle to placement. The pressure drop required to force a viscous sealant fluid through the wellhead dictates that placement rates (flow rate of the Portland cement) will be low. The cement particles in a slurry flowing through large-diameter horizontal pipe at low flow rate experience low shear rate. At low shear rate, cement solids in the slurry settle out from the flow and gather along the low side of the pipe, creating a flow obstruction. This obstruction increases pump pressure even to the point that displacement, i.e., continued flow through the flow line, is not possible. The deposited solids can also impede or interrupt the travel of any mechanical device used to sweep debris from the pipe ahead of cement. Finally, cement set retardation to allow extremely long placement time (maybe as long as 48 hours pumping) while remaining fluid and pumpable at surface conditions, temperature at the sea bed, and bottom hole temperature is complicated by effects of shear and fluid flow on the cement set retardation mechanism.